1. Field of the Invention
The present invention relates generally to integrated circuits and, more particularly, to integrated circuits implementing CMOS differential amplifiers.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As most people are aware, an integrated circuit is a highly miniaturized electronic circuit that is typically designed on a semiconductive substrate. Over the last 10 years, considerable attention has been paid to designing smaller, lower-power integrated circuits. These smaller, lower-power integrated circuits are often used in portable electronic devices that rely on battery power, such as cellular phones and laptop computers. As circuit designers research new ways to lower the power consumption of integrated circuits, they are constantly confronted with new challenges that need to be overcome in order to create the integrated circuits that will be part of the next generation computer, cellular phone, or camera.
The fundamental building block of the modern integrated circuit is the transistor. Transistors are generally fabricated on a semiconductive substrate, such as a silicon substrate. Silicon transistors are created by altering the electrical properties of silicon by adding other materials called “dopants” to the silicon. This process is known as doping. In n-type doping, dopants are added to the silicon that provide extra electrons that do not bond with the silicon. These free electrons make n-type silicon an excellent conductor. In p-type doping, silicon is doped with elements that cause an empty space, known as a “hole,” to develop in the silicon. Because these holes readily accept electrons from other silicon atoms, p-type silicon is typically also a good conductor.
Even though p-type silicon and n-type silicon are each good conductors, they are not always good conductors when joined. These junctions, called “p-n junctions,” are essentially one way streets for current—allowing it to flow in one direction across the junction but not in the other direction. When current can flow across the p-n junction, it is said to be “forward-biased,” and when current cannot flow across the p-n junction, it is considered to be “reverse-biased.”
A transistor is created by combining two p-n junctions. For example, a transistor might be arranged as either NPN or PNP. In this arrangement, a relatively small current (or voltage, depending on the type of transistor) applied to the center layer will essentially “open up” the transistor and permit a much greater current to flow across the transistor as a whole. In this fashion, transistors can act as switches or as amplifiers.
While there are numerous types of transistors, metal-oxide semiconductor field-effect transistors (“MOSFETs”) have been particularly popular over the past few years. One example of this type of MOSFET is known as an n-channel enhancement type MOSFET or NMOS transistor. The NMOS transistor is created by forming two heavily doped n-type regions in a p-type semiconductive substrate (i.e. NPN). These two n-type regions form regions known as the source and drain regions. Next, a thin layer of an oxide insulator may be grown on the surface of the substrate and metal, or another conductor, may be deposited on this oxide to create a gate region. Terminals are then attached to the source region, the drain region, and the gate region to create a semiconductor device with three terminals: the source (“S”) terminal, the drain (“D”) terminal, and the gate (“G”) terminal.
A voltage Vgs placed between the gate terminal and the source terminal of the NMOS transistor will create an electrical field in the semiconductive substrate below the gate terminal. This electrical field causes mobile electrons in the source region, the drain region, and the substrate to accumulate and form an n-type conductive channel in the p-type substrate. This conductive channel is known as the “induced channel.” This n-type induced channel effectively connects the drain and source regions together and allows a current, Id, to flow from the drain to the source (i.e. opening up the transistor). The voltage Vgs that is sufficient to cause enough electrons to accumulate in the channel to form an induced channel (i.e. to open up the channel) is known as the threshold voltage or Vth.
A transistor operating with a voltage Vgs less than the threshold voltage Vth is considered to be in the cut-off region because little or no current is able to flow between the drain and the source of the transistor. In many applications, it is preferable that the transistor not be in the cut-off region. One method of keeping a transistor out of the cut-off region is to apply a voltage Vgs to the transistor. This process is referred to as biasing. Two methods of biasing a transistor are self-biasing and fixed biasing. A transistor that has been self-biased typically has its gate terminal coupled to either its own drain terminal or to the terminal of another transistor located somewhere else in the circuit. A fixed biased transistor, on the other hand, is typically coupled to a voltage source either directly or through a resistor. In many digital applications, self-biasing is preferred because it is typically results in a more symmetrical digital output.
The voltage Vgs is not the only voltage that affects the flow of current between the drain region and the source region. A voltage Vds applied between the drain region and source region will appear as a voltage drop across the length of the induced channel. This means that if the voltage Vds is applied, the voltage along the induced channel may vary from the voltage Vgs at the source terminal to the voltage Vgs minus Vds at the drain terminal. This voltage change along the length of the induced channel may create a channel that is not a uniform depth. This variation in channel depth can affect the operation of the transistor. For instance, when the voltage Vds is less than the voltage Vgs minus Vth, the depth of the channel (and thus the current through the channel, Id) changes greatly as the voltage Vds changes. Under these conditions, the transistor is operating in a state known as “triode.” A transistor operating in the triode state may be referred to as a transistor in the triode region.
However, when the voltage Vds is greater than or equal to the voltage Vgs minus Vth, the current Id is unaffected by changes in the voltage Vds. This state is known as saturation, and a transistor operating in this state is considered to operating in the saturation region. The voltage Vds at which a transistor enters the saturation region is known as the saturation voltage. Because the voltage Vds to Id relationship is more stable in the saturation region than in the triode region, it may be preferable to operate a transistor in the saturation region when using the transistor as an amplifier.
A related type of MOSFET, known as p-channel enhancement type MOSFET or PMOS, is created on an n-type substrate with source and drain regions composed of p-type regions (i.e. PNP). PMOS transistors operate very similarly to NMOS transistors except that the threshold voltage is negative (i.e. positive between the source terminal and the gate terminal) and current flows from the source terminal to the drain terminal. Both PMOS and NMOS transistors may be used in circuits that employ Complementary MOS (“CMOS”) technology. Because CMOS technology allows circuit designers to employ both NMOS and PMOS transistors, it is one of the primary circuit design technologies in use today.
CMOS transistors (i.e. NMOS and PMOS transistors) can be used in a wide variety of amplifiers and switches. One such use is as a differential amplifier. The differential amplifier is one of the most widely used components in analog circuits. Among other things, it is typically used in CMOS input buffers, in some types of video amplifiers, and in balanced line receivers for digital data transmission. CMOS differential amplifiers have been an important part of the rapid growth of CMOS technologies over the past few years.
Generally, a differential amplifier has two voltage inputs, referred to as Vref and Vin, and one voltage output, referred to as Vout. Each of the inputs of the differential amplifier is sensitive to the other input. If Vin is greater than Vref, then Vout may be a first voltage level. If, however, Vref is greater than Vin, Vout may be a second voltage level (typically a higher voltage level). This relationship permits the differential amplifier to “detect” the voltage relationship between Vref and Vin. More specifically, a typical MOSFET differential pair consists of two NMOS transistors or two PMOS transistors. An input voltage Vref may be coupled to the gate terminal of one of these transistors and an input voltage Vin may be coupled to the gate terminal of the other transistor. The differential pair may be typically coupled to a tail current source. If Vin is greater than Vref, the increased voltage at the gate terminal of Vin transistor will lower the amount of current that can flow through that Vin transistor compared to the amount of current that can flow through the Vref transistor. When this happens, the current from the tail current source may not divide evenly, and this difference in tail current may result in a Vout at a low voltage level. Alternatively, if Vref is greater than Vin, the amount of current that can flow through the Vref transistor will be lower than the amount of current that can flow through the Vin transistor, which may result in a Vout at a high voltage level.
A transistor may be used as the tail current source in the MOSFET differential pair discussed above. If the tail current source transistor is not biased properly, current conduction through the transistor may be reduced or eliminated and current levels through the induced channel may be unstable. Disadvantageously, if the tail current source is improperly biased, the differential amplifier may not function properly. Tail current source transistors are typically biased using the self-biasing techniques previously described. These self-biasing techniques, however, may not be effective at the low supply voltage levels that are typically used in many modern, low-power devices.
Embodiments of the present invention may address one or more of the problems set forth above.